There is known a transflective LCD device including a reflective area and a transmissive area in each pixel of the LCD device. If the transmissive LCD device uses a lateral-electric-field mode, such as an IPS (in-plane-switching) mode or FFS (fringe-field-switching) mode, in the transmissive area, and a longitudinal electric field in the reflective area, there is a problem wherein the reflective mode represents a bright state at any time irrespective of whether or not a voltage is applied to a liquid crystal LC) layer in the reflective area, although the transmissive mode functions in a normally black mode.
Patent Publication JP-2003-344837A describes a technique for solving the above problem by adopting the longitudinal electric field in the reflective area and a specific structure, wherein the angle between the polarizing axis of a polarizing film and the optical axis of the LC layer as viewed from the side of counter substrate is set at 45 degrees, differently from the normal zero degree or 90 degrees generally adopted. In this structure, however, the transmissive mode is involved with a problem that the polarized state of light is changed in the internal of the LC layer and thus has a poor image characteristic upon display of a dark state in the transmissive area.
If the lateral-electric-field mode is adopted in both the transmissive area and reflective area, there is a problem known as a black-white inversion problem wherein the reflective area assumes a normally white mode whereas the transmissive area assumes a normally black mode in a typical driving scheme. The technique for solving the black-white inversion problem by adopting a retardation film only in the reflective area is described in Patent Publications JP-2005-338256A, -2006-171376A, -2006-71977A, and -2006-139286A. This technique roughly includes two types.
The first type is such that the reflective mode uses a lateral-electric-field mode, the retardation film is disposed on the side of counter substrate and has a retardation corresponding to that of a half-wavelength film (λ/2 film, λ is the wavelength of light), the LC layer has a retardation corresponding to that of a quarter-wavelength film (λ/4 film), and the reflection film is disposed at rear side of the LC layer, whereby the reflective mode operates in the normally black mode.
The second type is such that the reflective mode uses a lateral-electric-field mode, the counter substrate is not provided with a retardation film, the LC layer acts as a λ/2 film, a λ/4 film is disposed at the rear side of the LC layer, and the reflection film is disposed at the rear side of the λ/4 film, whereby the reflective area functions in the normally black mode. In both the first and second techniques, the combination of LC layer and retardation film acts as a wideband λ/4 film upon display of a dark state.
JP-2005-338256A introduces the λ/2 film in the reflective area to solve the above black-white inversion problem. More specifically, the IPS-mode transmissive LCD device using the lateral-electric-field includes a polarizing film that covers the entire pixel area as in the case of a transmissive LCD device, a retardation film having a retardation of λ/2 in the reflective area, and a LC layer having a retardation of λ/4 in the reflective area.
JP-2007-41572A describes a LCD device wherein the reflective mode uses a first gray-scale level signal, and the transmissive mode uses a second gray-scale level signal which is obtained by inverting the first gray-scale level signal, to solve the black-white inversion problem. This technique is referred to as a signal-inverting drive scheme, and the relationship between both the drive signals is referred to as an inverted-polarity relationship. FIG. 12 shows the configuration of a pixel in the LCD device described in this patent publication. The pixel 50 includes a reflective area 51 which includes a first pixel electrode 55 and a first common electrode 53, and a transmissive area 52 which includes a second pixel electrode 56 and a second common electrode 54. The liquid crystal (LC) layer in the reflective area 51 is driven by an electric field generated between the first pixel electrode 55 and the first common electrode 53, whereas the LC layer in the transmissive area 52 is driven by an electric field generated between the second pixel electrode 56 and the second common electrode 54. The first and second pixel electrodes 55, 56 are applied with the same pixel signal through respective thin film transistors (TFTs).
In the LCD device of FIG. 12, a first common-electrode signal, which is applied to the first common electrode 53 in the transmissive area 51, is inverted to generate a second common-electrode signal, which is applied to the second common electrode 54 in the transmissive area 52, to thereby uses a signal-inverting drive scheme. In this configuration, the LC layer in the reflective area 51 is applied with 5V, whereas the LC layer in the transmissive area 52 is applied with 0V. Thus, the optical axis or major axis of LC molecules in the LC layer are turned only in the reflective area 51 by the applied voltage, to solve the black-white inversion problem.
In the techniques described in JP-2007-41572A and -2005-338256A, the transmissive mode uses a lateral electric field and the major axis of the LC molecules in the LC layer is parallel or perpendicular to the optical axis of the polarizing film, that is, the optical axis of the LC layer has no effective angle with respect to the incident linearly-polarized light. In this case, the optical axis of the LC layer does not change the polarized state of the linearly-polarized light after passing through the LC layer, whereby the incident light and emitted light remain in the linearly-polarized state irrespective of the retardation of the LC layer. Thus, if the optical axis of the polarizing film disposed at the light emitting side is set perpendicular to the emitted light, the LC layer represents a dark state irrespective of the retardation of the LC layer. That is, the dark state obtained by the LCD device has a lower viewing angle dependency, lower chromaticity dispersion, and less dependency of the gap distance of the LC layer.
On the other hand, in the reflective area of the LCD device described in JP-2007-41572A and -2005-338256A, the major axis of the LC molecules is 45 degrees deviated from the polarized direction of the incident light upon display of a dark state, and the LC layer has a retardation of λ/4 and thus acts as a λ/4 film. In this configuration, the incident linearly-polarized light is changed to a circularly-polarized light by the function of the LC layer and reflection film to represent the dark state. The two publications use different techniques at this stage. In the technique described in JP-2005-338256A, the λ/2 film is disposed only in the reflective area to rotate the incident light to achieve the 45 degrees between the incident light and the major axis of the LC molecules only in the reflective area. In the technique described in JP-2007-41572A, the driving scheme rotates the LC molecules only in the reflective area by 45 degrees to achieve an angle of 45 degrees between the incident light and the major axis of the LC molecules only in the reflective area.
As described heretofore, in the techniques described in JP-2007-41572A and -2005-338256A, the LC molecules in the LC layer in the reflective area have a major axis which is 45 degrees deviated away from the polarized direction of the linearly-polarized incident light upon display of a dark state. This causes a change of the polarized state of the linearly-polarized light within the LC layer, and thus the dark state can be achieved after the incident light reaches the reflection film and turned to a circularly polarized light. Accordingly, the incident light is subjected to wavelength dispersion by the LC layer due to the birefringence thereof, which depends on the wavelength of the incident light as well as the gap distance of the LC layer, whereby the dark state achieved in the reflective area has a viewing angle dependency and chromaticity dispersion. In addition, the gap distance dependency causes fluctuation of the black luminance to degrade the contrast ratio upon display of the dark state. Thus, although the lateral-electric-field-mode transflective LCD device is superior, in the performance such as the contrast ratio and viewing angle dependency, to the longitudinal-electric-field-mode transflective LCD devices in the transmissive mode, the lateral-electric-field-mode transflective LCD device is inferior in the reflective mode to those longitudinal-electric-field-mode LCD devices in the performance such as the contrast ratio.
In the LCD device wherein the reflective area operates in a longitudinal-electric-field mode and the transmissive area operates in a lateral-electric-field mode, as described in JP-2003-344837A, if the ordinary optical system is adopted, the LC layer does not perform an ON/OFF operation in the reflective mode, although the transmissive mode effectively functions as a normally block mode. In addition, if 45 degrees is adopted as the angle between the optical axis of the polarizing film and the optical axis of the LC layer as observed normal to the substrate, the transmissive area assumes a normally white mode, whereby the polarized state of the incident light is changed within the internal of LC layer upon display of a dark state, thereby causing a lower contrast ratio in the transmissive area.
As described heretofore, the transflective LCD devices described in the patent publications have the common problem of lower contrast ratio at least in one of the transmissive area and reflective area.